Studies from many laboratories now generally support the amyloid (or Abeta) hypothesis of Alzheimer's disease (AD), and therapeutic trails based on this concept have begun. Nevertheless, we are left with many unresolved and fascinating biological questions about how a small hydrophobic peptide slowly accumulates in the brain with age and can apparently initiate cytopathology. Perhaps foremost among these questions is exactly why cerebral Abeta levels are elevated in humans who develop AD. Increased Abeta production appears to explain only a small minority of cases, principally those inheriting mutations in APP or the presenilins. This realization suggests that many, perhaps most, cases of AD are caused by faulty clearance of a peptide that is produced at normal levels throughout life. Our proposed work here is based on 3 related hypotheses: 1) defects in proteases which degrade Abeta may underlie some or many cases of familial and sporadic late-onset AD; 2) non-proteolytic clearance mechanisms (e.g., receptor-mediated efflux through the blood brain barrier) could explain the rise in cerebral Abeta in other cases; and 3) whether or not these two mechanisms are actually causative, subtly activating such proteases or else stimulating protein transport mechanisms could lower cerebral Abeta levels therapeutically. To extend our productive work under this grant on both the basic and applied aspects of this understudied subject, we propose 3 aims: Aim 1: To determine whether central or peripheral expression of engineered versions of two well-characterized Abeta degrading proteases - neprilysin and insulin degrading enzyme (IDE) -can safely and effectively decrease cerebral Abeta burden in mouse models of AD. Aim 2: In view of the recent unequivocal demonstration that IDE exists normally on the plasma membrane (namely, as the receptor for varicella-zoster virus), to further ellucidate the unusual cell biology and membrane trafficking of IDE that enables it to be involved in the degradation of both extracellular/intraluminal substrates (e.g., Abeta, insulin, amylin) and cytoplasmic substrates (e.g., AICD). Aim 3: To pursue a recently initiated project that addresses an even less well-studied aspect of Abeta clearance than proteolysis, namely, to rigorously identify and validate the cell- surface receptors capable of mediating the efflux of soluble Abeta across the BBB, using both in vitro and in vivo models. Understanding how both proteolysis and non-proteolytic clearance regulate Abeta levels in the brain has major implications for both the genesis and treatment of this complex disorder. At the same time, the proposed experiments have fundamental implications for peptide turnover in the mammalian brain and for the cell biology of metalloproteases. Many scientists now support the idea that the gradual buildup of a small protein, amyloid beta-protein (Abeta), in brain regions serving memory and thinking causes Alzheimer's disease. This grant will examine exactly why this buildup occurs over time, focusing on two potential problems: 1) faulty cutting up of the Abeta protein in the brain; or 2) faulty transport of the Abeta protein from the brain into the circulation. We will also explore new ways to increase the cutting up or the transport of Abeta as future therapeutic approaches for preventing Alzheimer's disease.